Investigators propose a double-blind, placebo-controlled cross-over study of 20 patients currently awaiting initiation of ivacaftor therapy. Patients enrolled in the study will be asked to undergo screening (day -28), and baseline assessments (day 0) and re-assessment studies (day 28) after treatment period 1, followed by baseline assessment at the beginning of treatment period 2 (day 56) and at the conclusion of treatment period 2 (day 84). A further assessment (day 224) will be performed 140 (+/-7) days following commencement on open label Ivacaftor.